The sowers of today's confusion, it turns out, were luxury imports such as Audi, Mercedes and BMW. Beginning in the 1970s, their alphanumeric names persuaded American consumers that inscrutable designations like 300 SEL and A8L added prestige worth paying for.

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As important, makers of luxury cars found it cheaper and more effective to advertise one brand than many models, explained Nacef Mouri, a marketing professor at George Mason University in Fairfax, Va.

"Giving cars numbers is done when you want the consumer to focus on the actual brand," Mouri said.

Not just any letter will do, however, as the litter of x's on trunk lids suggests.

Part of the mystique is that "x" traditionally designates experimental technologies, suggesting daring and newness. Another part is that the letter x is a plosive, said Nina Beckhardt, a professional brand-namer: a letter that, when pronounced (ecks), creates a small explosion in the ears of listeners that gets their attention and engages them.

"When you're trying to make a strong statement (about a product) or have a strong name, a letter with a propulsive feeling, a plosive, can accomplish that," said Beckhardt, the president and creative director of Namebase, a branding firm in New York.

The statement's way stronger when carmakers use letters and numbers in a code that spells prestige.

"Someone who is buying a 7-series BMW is buying that knowing it is, logically and actually, a higher caliber car than the 3- or 5-series," Beckhardt said. "Also, everyone else around him knows that it is of the highest caliber. So, when these alphanumeric names are viewed in relation to one another, they can hold a lot of weight in terms of status and communication of wealth."

Terry Whitesides, 62, a Honda Civic driver from San Diego, said that he knows the how the luxury car codes work. "Whether you're the owner or the envious, you don't even have to say anything. You know where you stand with people you've never met before," he said.

Unfortunately, the same goes for Mercedes owners, some discover, after they've bought low-end C-series models.

"If you're a C-class owner, you're a second-citizen, never mind that some of us think $32,300 is a lot of money to pay for a car," an owner named JJ complained on a Mercedes discussion board.

Even for consumers who don't know the codes, Mouri said, alphanumeric names "created this implicit differentiation, with high quality cars having letters and numbers and low quality cars having names."

The problem for many U.S. automakers is that branding models with numbers and consonants doesn't make them any better. Pontiac's G6-GXP, a mid-size sedan, for example, has all the glamour of a rental car lot, and GM is eliminating the Pontiac brand.

Brighter days are dawning, maybe. Namebase's Beckhardt, who says alphanumeric names are passe, is bringing back pronounceable ones that "clearly communicate the benefits or personality of the car."

A fave is the name "Picanto," which Namebase named for Kia, the Korean carmaker.

The name, according to Beckhardt, says, among other things, "compact," "zippy" and "spicy."